This proposal is to continue our studies of brain aging in rhesus macaques that have been calorically restricted since 1989 or 1994. We will investigate processes and mechanisms that may explain why caloric restriction (CR) exerts a salutary effect on the brain. The key hypothesis is that a metabolic shift occurs due to CR. In this study we will employ a powerful array of investigative tools: MRI to examine longitudinal change in brain morphology, FDG PET to examine glucose metabolic function, detailed cognitive and behavioral testing to examine executive, memory and fine motor dexterity, and on the animals that have died due to natural causes throughout the course of the long-running study, we will examine the brain in fine detail for neuropathologic features and for key indicators of metabolic changes. Aim 1 of this study examines longitudinal change on brain MRI including volumetric changes, and iron deposition changes. In addition we will employ newer scan sequences to obtain greater characterization of white and gray matter. Aim 2 will examine cognitive and motor function using an already existing touch screen response system. Aim 3 examines fine-grained neurohistopathology features including histological characterization of beta amyloid, tau, p-tau, synaptic density, and reactive astrocytes (GFAP). Aim 4 examines the central hypothesis that CR induces an upregulated state of energy metabolism in the brain assessed in vivo with PET FDG and in situ with assays of key metabolic regulators PGC-1a, SIRT1, mTOR, and AMPK. Finally Aim 5 provides an integrative Aim examining associative convergence among the various markers in the study that would lead us to conclude that CR retards the aging process in multiple domains. The significance of this project is quite high since body weight is a modifiable risk factor for disease. In order for CR or CR mimetics to be applied in humans it is critical to understand the effect of CR on the brain in a primate model. In this project we will attain a comprehensive and completely novel set of data on the long-term (18-23 years) effects of CR on the brain and behavior. The powerful combination of the metabolic and structural imaging, cognitive assessment and in-situ brain assays obtained in this multidisciplinary and translational project will lead to better understanding of the mechanisms by which CR affects the brain.